Applications Area D. Wing
Internet-Draft A. Yourtchenko
Intended status: Standards Track P. Natarajan
Expires: February 21, 2011 Cisco
August 20, 2010
Happy Eyeballs: Trending Towards Success (IPv6 and SCTP)draft-wing-http-new-tech-01
Abstract
People like their computers to work quickly. During the transition
to new technology, both old and new technologies have to peacefully
co-exist. However, if users experience connection delays attributed
to the new technology the new technology will be shunned.
HTTP ("The Web") is one of the most visible and time-critical
applications that is used by nearly every Internet user. It is
critical that new technologies which improve HTTP not impair or delay
the display of HTTP content. It is also important that users retain
the ability to share URIs amongst friends and colleagues, even if the
other users have not upgraded to the new technology.
This draft makes several recommendations to ensure user satisfaction
and a smooth transition from HTTP's pervasive IPv4 to IPv6 and from
TCP to SCTP.
The audience for this draft is application developers and content
providers. This draft is discussed on the Applications Discuss
mailing list, https://www.ietf.org/mailman/listinfo/apps-discuss.
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
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The list of current Internet-Drafts can be accessed at
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This Internet-Draft will expire on February 21, 2011.
Copyright Notice
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Internet-Draft Happy Eyeballs: Trending Towards Success August 20101. Introduction
In order to use HTTP successfully over IPv6 or SCTP, it is necessary
that the user enjoys nearly identical performance as compared to
their old technology (IPv4 and TCP). A combination of today's
applications, IPv6 tunneling and IPv6 service providers, IPv4 NAT,
and some of today's content providers all cause the user experience
to suffer (Section 3). For IPv6, Google ensures a positive user
experience by using a DNS white list of IPv6 service providers who
peer directly with Google [whitelist]. However, this is not scalable
to all service providers worldwide, nor is it scalable for other
content providers to operate their own DNS white list.
Instead, this document suggests a mechanism for applications to
quickly determine if IPv6, IPv4, SCTP, or TCP is the most optimal to
connect to a server. The suggestions in this document provide a user
experience which is superior to HTTP using TCP and IPv4, especially
in IPv6/IPv4 transition environment with dual stack hosts (e.g.,
[RFC4213], DS-Lite [I-D.ietf-softwire-dual-stack-lite], 6rd
[I-D.despres-6rd]).
Once a certain address family is successful, it trends towards
preferring that address family. Thus, repeated use of the
application DOES NOT cause repeated probes over both address
families.
The application recommendations in this document are primarily for
HTTP clients ("web browsers") and may also be helpful for other
applications.
2. Notational Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Problem Statement
As discussed in more detail in Section 3.1, it is important that the
same URI and hostname be used for IPv4, IPv6, SCTP, and TCP. Using
separate namespaces causes namespace fragmentation and reduces the
ability for users to share URIs and hostnames, and complicates
printed material that includes the URI or hostname.
As discussed in more detail in Section 3.2, IPv6 connectivity is
sometimes broken entirely or, due to tunnel technologies might be
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slower than native IPv4 connectivity. However, due to port
limitations inherent in stateful IPv6/IPv4 translators [BEHAVE], it
is important that web browsers begin preferring IPv6 over IPv4 in
order to avoid those port limitations.
As discussed in more detail in Section 3.3, there is no standard
mechanism to indicate a host supports a non-TCP transport protocol,
such as SCTP.
3.1. URIs and hostnames
URIs are often used between users to exchange pointers to content --
such as on Facebook, email, instant messaging, or other systems.
Thus, production URIs and production hostnames containing references
to IPv4, IPv6, TCP, or SCTP will only function if the other party
also has application, OS, and a network that can access the URI or
the hostname.
3.2. IPv6
When IPv6 connectivity is impaired, today's IPv6-capable web browsers
incur many seconds of delay before falling back to IPv4. This harms
the user's experience with IPv6, which will slow the acceptance of
IPv6, because IPv6 is frequently disabled in its entirety on the end
systems to improve the user experience.
Reasons for such failure include no connection to the IPv6 Internet,
broken 6to4 or Teredo tunnel, and broken IPv6 peering. To prevent
this delay an experiment with IPv6 connectivity, content providers
use a separate namespace for their web server (e.g.,
ipv6.example.com), but doing that with production systems causes the
problems described in Section 3.1.
3.3. SCTP
SCTP provides benefits over TCP [I-D.natarajan-http-over-sctp].
Unlike IPv6 which has an AAAA record, there is no DNS query that
indicates a host supports SCTP [RFC4960], and HTTP URI scheme is not
extensible to support an SRV query that could provide such support.
Even if there was, it isn't possible to determine if a middlebox,
such as a firewall or a NAT, would block the SCTP association.
4. HTTP Client Recommendations
To provide fast connections for users, HTTP clients should make
connections quickly over various technologies, automatically tune
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itself to avoid flooding the network with unnecessary connections
(i.e., for technologies that have not made successful connections),
and occasionally flush its self-tuning.
If an HTTP client supports IPv6 and SCTP (in addition to IPv4 and
TCP), the procedures described in Section 4.1 and Section 4.2 are
performed together.
4.1. IPv6
This section details how to provide robust dual stack service for
both IPv6 and IPv4, so that the user perceives very fast application
response.
The HTTP client is configured with one value, P. A positive value
indicates a preference for IPv6 and a negative value indicates a
preference for IPv4. A value of 0 indicates equal weight, which
means the A and AAAA queries and associated connection attempts will
be sent as quickly as possible. The absolute value of P is the
measure of a delay before initiating a connection attempt on the
other address family. There are two P values maintained: one is
application-wide and the other is specific per each destination
(hostname and port).
The algorithm attempts to delay the DNS query until it expects that
address family will be necessary; that is, if the preference is
towards IPv6, then AAAA will be queried immediately and the A query
will be delayed.
The HTTP client starts two threads in order to minimize the user-
noticeable delay ("dead time") during the connection attempts:
thread 1: (IPv6)
* If P<0, wait for absolute value of p*10 milliseconds
* send DNS query for AAAA
* wait until DNS response is received
* Attempt to connect over IPv6 using TCP
thread 2: (IPv4)
* if P>0, wait for p*10 milliseconds
* send DNS query for A
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* wait until DNS response is received
* Attempt to connect over IPv4 using TCP
The first thread that succeeds returns the completed connection to
the parent code and aborts the other thread (Section 5.2).
After a connection is successful, we want to adjust the application-
wide preference and the per-destination preference. The value of P
is incremented (decremented) each time an IPv6 (IPv4) connection is
successfully made. When a connection using the less-preferred
address family is successful, it indicates the wrong address family
was used and the P is halved:
o If P>0 (indicating IPv6 is preferred over IPv4) and the first
thread to finish was the IPv6 thread it indicates the IPv6
preference is correct and we need to re-enforce this by increasing
the application-wide P value by 1. However, if the first thread
to finish was the IPv4 thread it indicates an IPv6 connection
problem occurred and we need to aggressively prefer IPv4 more by
halving P and rounding towards 0.
o If P<0 (indicating IPv4 is preferred over IPv6) and the first
thread to finish was the IPv4 thread it indicates the preference
is correct and we need to re-enforce this gently by decreasing the
application-wide P value by 1. However, if the first thread to
finish was the IPv6 thread it indicates an IPv4 connection problem
and we need to aggressively avoid IPv4 by halving P and rounding
towards 0.
o If P=0 (indicating equal preference), P is incremented if the
first thread to complete was the IPv6 thread, or decremented if
the first thread to complete was the IPv4 thread.
After adjusting P, it should never be larger than 4 seconds -- which
is similar to the value used by many IPv6-capable HTTP clients to
switch to an alternate A or AAAA record.
Note: Proof of concept tests on fast networks show that even
smaller value (around 0.5 seconds) is practical. More extensive
testing would be useful to find the best upper boundary that still
ensures a good user experience.
4.2. SCTP
Due to the proliferation of NATs on the IPv4 Internet the best
success for SCTP can be achieved by attempting both native SCTP
connections and SCTP-over-UDP [I-D.tuexen-sctp-udp-encaps]
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connections.
For SCTP the following parameters are used:
SWAIT: Application-wide wait time for an SCTP association attempt to
complete. Default value of 50ms is RECOMMENDED.
PREF: This denotes per-destination transport preference. Possible
values are "TCP", "SCTP", and "BOTH". Default value of
"BOTH" is RECOMMENDED.
The HTTP client starts several threads in order to minimize the user-
noticeable delay ("dead time") during the connection attempts. The
client starts one or more threads based on the following logic:
If ((PREF == BOTH) or (PREF == SCTP)) start thread 1. If making a
connection using IPv4 start thread 2.
If ((PREF == BOTH) or (PREF == TCP)) start thread 3.
thread 1 (SCTP):
* Attempt to connect using SCTP (i.e., send SCTP INIT)
thread 2 (SCTP over UDP):
* Attempt to connect using SCTP over UDP (i.e., send SCTP INIT
over UDP)
thread 3 (TCP):
* Attempt to connect using TCP
If an SCTP association attempt was made by a thread, the HTTP client
waits for at least K ms; K = max(SWAIT, time taken for the TCP
connection to complete). If the TCP connection finishes during this
wait period, the HTTP client MAY choose TCP for the current HTTP
transfer but MUST wait until K ms to figure if the SCTP association
can be completed.
If the HTTP client did not choose TCP during the wait period and the
SCTP association completes successfully, the HTTP client prefers SCTP
over TCP connections and abandons the TCP connection.
After a connection is successful, we want to adjust the per-
destination preference for this destination. It is not recommended
to dynamically adjust the application-wide default value for SWAIT.
If the SCTP association was successful, set destination's
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PREF="SCTP", else set PREF="TCP".
5. Additional Considerations
This section discusses considerations and requirements that are
common to new technology deployment.
5.1. Additional Network and Host Traffic
Additional network traffic and additional server load is created due
to these recommendations and mitigated by application-wide and per-
destination timer adjustments. The intent of this document is to
show how good user experience can be maintained while the
transitioning from IPv4 to IPv6, and transitioning from TCP to SCTP.
The good user experience is to the benefit of the user but to the
detriment of the network and server that are serving the user.
5.2. Abandon Non-Winning Connections
It is RECOMMENDED that the non-winning connections be abandoned, even
though they could be used to download content. This is because some
web sites provide HTTP clients with cookies (after logging in) that
incorporate the client's IP address, or use IP addresses to identify
users. If some connections from the same HTTP client are arriving
from different IP addresses, such HTTP applications will break.
Editor's note: If we can provide guidance to IPv6 and SCTP
developers that connections from the same client could arrive on
IPv4, IPv6, TCP, and SCTP we could eliminate the above paragraph.
But could we be sure all web sites would follow such guidance?
5.3. Flush or Expire Cache
Because every network has different characteristics (working or
broken IPv6 connectivity, middlebox that permits or blocks SCTP,
etc.) the IPv6/IPv4 preference value (P) and the SCTP parameters
(SWAIT and PREF) SHOULD be reset to their default whenever the host
is connected to a new network. However, in some instances the
application and the host are unaware the network connectivity has
changed (e.g., when behind a NAT) so it is RECOMMENDED that per-
destination values expire after 10 minutes of inactivity.
5.4. Determining Address Type
[[[ IS THIS SECTION NECESSARY ??
For some transitional technologies such as a dual-stack host, it is
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easy for the application to recognize the native IPv6 address
(learned via a AAAA query) and the native IPv4 address (learned via
an A query). For other transitional technologies [RFC2766] it is
impossible for the host to differentiate a transitional technology
IPv6 address from a native IPv6 address (see Section 4.1 of
[RFC4966]). Replacement transitional technologies are attempting to
bridge this gap. It is necessary for applications to distinguish
between native and transitional addresses in order to provide the
most seamless user experience.
]]]
5.5. DNS Behavior
Unique to DNS AAAA queries are the problems described in [RFC4074]
which, if they still persist, require applications to perform an A
query before the AAAA query.
[[Editor's Note: It is believed these defective DNS servers have
long since been upgraded. If so, we can remove this section.]]
5.6. Thread safe DNS resolvers
Some applications and some OSs do not have thread safe DNS resolvers,
which complicates implementation of simultaneous A and AAAA queries
for IPv4/IPv6.
5.7. Middlebox Issues
Some devices are known to exhibit what amounts to a bug, when the A
and AAAA requests are sent back-to-back over the same 4-tuple, and
drop one of the requests or replies [DNS-middlebox]. However, in
some cases fixing this behaviour may not be possible either due to
the architectural limitations or due to the administrative
constraints (location of the faulty device is unknown to the end
hosts or not controlled by the end hosts). The algorithm described
in this draft, in the case of this erroneous behaviour will
eventually pace the queries such that this issue is will be avoided.
The algorithm described in this draft also avoids calling the
operating system's getaddrinfo() with "any", which should prevent the
operating system from sending the A and AAAA queries on the same
port.
5.8. Multiple Interfaces
Interaction of the suggestions in this document with multiple
interfaces is for further study.
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Internet-Draft Happy Eyeballs: Trending Towards Success August 20106. Content Provider Recommendations
Content providers SHOULD provide both AAAA and A records for servers
using the same DNS name for both IPv4 and IPv6.
7. Security Considerations
[[Placeholder.]]
See Section 5.2.
8. Acknowledgements
The mechanism described in this paper was inspired by Stuart
Cheshire's discussion at the IAB Plenary at IETF72, the author's
understanding of Safari's operation with SRV records, Interactive
Connectivity Establishment (ICE [I-D.ietf-mmusic-ice]), and the
current IPv4/IPv6 behavior of SMTP mail transfer agents.
Thanks to Fred Baker, Jeff Kinzli, Christian Kuhtz, and Iljitsch van
Beijnum for fostering the creation of this document.
Thanks to Scott Brim and Stig Venaas for providing feedback on the
document.
9. IANA Considerations
This document has no IANA actions.
10. References10.1. Normative References
[I-D.tuexen-sctp-udp-encaps]
Tuexen, M. and R. Stewart, "UDP Encapsulation of SCTP
Packets", draft-tuexen-sctp-udp-encaps-05 (work in
progress), July 2010.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4960] Stewart, R., "Stream Control Transmission Protocol",
RFC 4960, September 2007.
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